Here's a small tip that might save you some time and frustration.
I needed to replace the breech seal on my son's EV2 Mk4. Damn thing kept disappearing down the barrel each I tried to seat it. Plugged the barrel with a couple of the VFG cleaning wads and the seal had nowhere else to go except the groove. Once in place I just fired them out, breech seal replacement and clean in one go.